1. Field of the Invention
The present invention relates to a mounting structure of a reed switch for detecting rotation of a magnet which is provided in an eddy-current indicator such as an eddy-current speed meter or tachometer for a vehicle.
2. Description of the Prior Art
Generally, an eddy-current indicator has a magnet which is rotated in proportion to the number of rotation of wheels or an engine of a vehicle, and an induction plate which is able to pivot with crossing a magnetic flux formed by the magnet. In the eddy-current indicator, eddy-current corresponding to the rotation of the magnet is generated in the induction plate, and the eddy-current further makes the induction plate pivoted in accordance with the rotation number of the magnet, so that a speed of the vehicle or rotation number of the engine can be indicated by a pivot angle of a pointer which is mounted on the induction plate.
In the eddy-current indicators as described above, there is known one indicator having a digital display device for indicating speed or rotation number in addition to the the analog display by the pointer. The digital display device comprises a reed switch as a rotation sensor which is arranged in the vicinity of the magnet. The reed switch generates pulse having a period corresponding to the rotation number of the magnet by an on-off control of the reed switch in accordance with magnetic fields generated by the rotation of the magnet, and then the pulse is processed into the digital form.
As a structure for mounting the above-mentioned reed switch in a housing holding therein the magnet rotatably, there is known a structure shown in FIGS. 1 to 5.
In these drawings, a housing 1 is formed of a resin material integrally molded. As shown in FIG. 1, the housing 1 has a concave portion 3 having a long and narrow opening 2 at the side of the central portion of the housing 1. In the concave portion 3 of the housing 1, an assembly body 5 having a reed switch 4 is provided such that the reed switch 4 is disposed in the opening 2.
As shown in FIGS. 2 and 3, the assembly body 5 includes an insulating plate 7 (distributing plate) having copper-leaf conductive bodies 6a, 6b at the top surface thereof. A pair of terminals 8a, 8b are secured at the distributing plate 7 with rivets 9a, 9b, respectively, and therefore the terminals 8a, 8b are electrically connected to the copper-leaf conductive bodies 6a, 6b, respectively. The reed switch 4 is arranged between side arm portions 7a, 7b of the distributing plate 7. The reed switch 4 has leg portions 4a, 4b which extend toward the opposite directions. The tip parts of the leg portions 4a, 4b are formed into a hook-shape, respectively, which is engaged with each of the side arms 7a, 7b of the distributing plate 7 from the underside of the side arms 7a, 7b. In this state, the leg portions are in contact with the under surfaces of the side arms. The ends of the hook-shape tip parts of the leg portions 4a, 4b are respectively secured to the the copper-leaf conductive bodies 6a, 6b by soldering at soldering points 15a, 15b as shown in FIG. 3. Thus, the reed switch 4 is electrically connected to the copper-leaf conductive bodies 6a, 6b.
The above-mentioned assembly body 5 is mounted in the concave portion 3 of the housing 1 as shown in FIGS. 4 and 5. Then, the reed switch 4 of the assembly body 5 is accommodated in the opening 2 of the housing 1, and further the terminals 8a, 8b are secured on a wall 1a of the housing 1 with screws 10a, 10b, respectively. In this mounting state, the reed switch 4 is disposed near a magnet (not shown) which is held rotatably in the housing 1. The magnet is connected to a rotation transmission rod (not shown) which passes through a bearing lb of the housing 1. To the transmission rod, a pointer is mounted.
Incidentally, in FIGS. 1 to 5, the reference numerals 11a, 11b denote holes which are formed in the concave portion 3 of the housing 1 to contain head portions 9c, 9d of the revets 9a, 9b therein. The reference numerals 12a, 12b denote holes formed in the wall la of the housing 1 for mounting the screws 10a, 10b therein which pass through screw holes 13a, 13b formed in bending vertical portions of the terminals 8a, 8b. The reference numerals 21a, 21b are recess portions formed in both sides of the opening 2 of the concave portion 3, in which the leg portions 4a, 4b of the reed switch 4 are contained respectively so as not to directly contact with the housing 1.
In the above-mentioned structure, there is a disadvantage that when the reed switch 4 is subjected to an on-off control, vibration is caused by the operation of the reed switch and the operation vibration energy is transmitted to not only the side arms 7a, 7b of the distributing plate 7 but also the housing 1 through the leg portions 4a, 4b of the reed switch 4. This is because the leg portions 4a, 4b are in contact with the side arms 7a, 7b of the distributing plate 7 at the under side of the side arms. The transmitted vibration generates a strong resonance sound (an operation vibration sound of the reed switch) when the rotation of the magnet reaches at a certain number.
FIG. 6 shows frequency characteristics of the operation sound generated in an eddy-current indicator which does not have a reed switch at a certain car speed, and FIG. 7 shows frequency characteristics of the operation sound generated in an eddy-current indicator which has the reed switch mounted by the above-mentioned mounting structure. As apparently seen from the drawings, when the reed switch is provided, a strong resonance sound having a peak at about 4 kHz is generated, which is not seen in the case that the reed switch is not provided.